1. Field of the Invention
The present invention relates to a radio receiver and, particularly, to a radio receiver of the MB-OFDM system that absorbs a delayed wave due to multipath by inserting a no-signal interval, such as zero suffix or zero prefix, into a transmission signal.
2. Description of Related Art
In the radio communications, techniques of inserting a guard interval into a transmission signal and performing adaptive equalization at a receiver end are used in order to reduce the effects of multipath fading.
For example, in UWB (Ultra Wide Band), which is the short distance radio communication technology that is standardized as ECMA-368 by ECMA (European Computer Manufacturer Association) and uses MB-OFDM (Multi-band Orthogonal Frequency Division Multiplexing) for the PHY layer, the zero suffix (ZS) containing 37 samples is inserted after the OFDM symbol containing 128 samples, which is generated by IFFT (Inverse Fast Fourier Transform) as shown in FIG. 8. A UWB transmitter superimposes the transmission data containing the zero suffix upon a carrier wave and transmits it. Because the UWB uses the frequency hopping technology, the UWB transmitter uses three carrier waves with different frequencies and transmits data by a carrier wave which is periodically changed for each OFDM symbol.
Receiving the signal containing the zero suffix, the UWB receiver adds the data of the zero suffix interval, which is inserted after the OFDM symbol, to the top of the OFDM symbol. Such adding process is referred to hereinafter as “overlap-addition”. The overlap-addition performed in the UWB receiver compensates the effects of the multipath fading in a transmission path between the UWB transmitter and the UWB receiver. Five samples of time out of the 37 samples of zero data which are inserted as the zero suffix is assigned as a time to switch a carrier frequency in the receiver. Thus, the number of samples used for the overlap-addition is 32.
FIG. 9 shows the configuration of an OFDM receiver 7 according to a related art. The receiving operation of the MB-OFDM receiver 7 is briefly described hereinafter with reference to FIG. 9. A signal which is received by an antenna 11 is band-selected by a band pass filter (BPF) 12 and then amplified by a low noise amplifier (LNA) 13. The BPF 12 is a filter to select a band group to receive from a plurality of band groups of MB-OFDM to thereby remove out-of-band noises and interference waves. The signal which is amplified by the LNA 13 is quadrature-demodulated by a quadrature demodulator 14. Because the frequency hopping is performed in the MB-OFDM system, a local frequency fc which is generated by an oscillator (not shown) and input to the quadrature demodulator is periodically switched according to a frequency hopping pattern.
A baseband signal which is demodulated by the quadrature demodulator 14 is input to a low pass filter (LPF) 15, so that its high frequency component is filtered out. The signal is then amplified to a prescribed signal level by a variable gain amplifier (VGA) 16.
An A/D converter (ADC) 17 performs sampling and quantization of the baseband signal which is amplified by the VGA 16 and outputs a discrete digitized baseband signal. The output signal of the ADC 17 is input to a synchronous processing unit 18.
The synchronous processing unit 18 captures the OFDM signal symbol synchronous timing and the frame synchronous timing. Further, it removes a preamble and rotates the phase of the input baseband signal so as to correct the phase error between the carrier wave frequency of the received signal and the local frequency used for the quadrature demodulation. The synchronous processing unit 18 includes a correlator (not shown) for calculating a correlation value between an input signal and a known preamble signal and determines the OFDM signal symbol synchronous timing based on the peak of the correlation value calculated by the correlator.
An overlap-adder 79 removes the zero suffix interval from the input baseband signal and adds the 32 samples of data in the zero suffix interval to the top of the OFDM symbol interval.
A FFT unit 20 performs fast Fourier transformation on the baseband signal after the overlap-addition and outputs demodulated data for each subcarrier.
A subcarrier decoder 21 performs frequency equalization, deinterleaving, Viterbi decoding, descrambling or the like on the demodulated data for each subcarrier using pilot tone, and outputs the decoded data.
The process of the overlap-addition performed in the overlap-adder 79 and its problem are described hereinafter. FIG. 10A is a conceptual diagram showing a received signal of the OFDM receiver 7. It shows the signals which are reached through three transmission paths 1 to 3. The transmission path 1 corresponds to a transmission channel of a direct wave, and the transmission paths 2 and 3 correspond to transmission channels of a reflected wave (delayed wave). Thus, the signals received through the transmission paths 2 and 3 are delayed with respect to the signal received through the transmission path 1.
In order to correct the distortion of a received signal due to the existence of such a delayed wave, the overlap-adder 79 performs overlap-addition. FIG. 10B is a conceptual diagram of the overlap-addition process. Specifically, the process adds the 32-sample data of the zero suffix interval (ZS interval) to the top of received data 82 in the 128-sample interval (FFT interval) which corresponds to 1 OFDM symbol from the top of the direct wave. A delayed part 821 of maximum 32 samples which is delayed behind the FFT interval and reached in the ZS interval is thereby added to the received data 82 in the FFT interval. The overlap-addition allows the correction of the distortion of a received signal caused by the existence of a delayed wave due to multipath.
The overlap-adder 79 performs addition represented by the following Expression 1. In Expression 1, Sn[k] indicates received data which is input to the overlap-adder 79, and Sout[k] indicates data after overlap-addition which is output from the overlap-adder 79. Thus, the overlap-adder 79 adds 32 samples of data Sn[128] to Sn [159] in the zero suffix interval to the top of the OFDM symbol Sn[0] to Sn[31], thereby generating 128 samples of output data Sout[0] to Sout[127] from 165 samples of input data Sn[0] to Sn[164]. Because 5 samples of data Sn[160] to Sn[164] are a guard interval which is saved for frequency hopping, they are not used for the overlap-addition.
                                          S            out                    ⁡                      [            k            ]                          =                  {                                                                                          Sn                    ⁡                                          [                      k                      ]                                                        +                                      Sn                    ⁡                                          [                                              k                        +                        128                                            ]                                                                                                                    k                  ∈                                      [                                          0                      ,                      31                                        ]                                                                                                                        Sn                  ⁡                                      [                    k                    ]                                                                                                k                  ∈                                      [                                          32                      ,                      127                                        ]                                                                                                          Expression        ⁢                                  ⁢        1            
FIG. 11 shows an example of the configuration of the overlap-adder 79. The overlap-adder 79 may include a shift memory 80 which is composed of 160 storage cells connected in series, and 32 adders 8100 to 8131.
In the process of overlap-addition, DC offsets and noises in the zero suffix interval are added to the OFDM symbol. For example, because 32-sample data is added to the 128-sample OFDM symbol in the OFDM receiver 7, a noise becomes (128+32)/128 times larger through the overlap-addition. Thus, the overlap-addition causes the penalty of 10 log10(160/128)=0.97 dB, so that the required CNR (Carrier to Noise Ratio) deteriorates by about 0.97 dB. Because 32 samples of data are always added regardless of the amount of delay spread in a UWB receiver of a related art, the penalty due to the overlap-addition occurs uniformly regardless of the amount delay spread. The delay spread is a parameter indicating the degree of spread of a signal transmitted through a multipath with respect to time direction, that is, delay characteristics. The delay spread is defined as a root-mean-square value (distribution) of the spread of power distribution of a received signal with respect to a delay time. Because the square root of the delay spread is also used as a parameter to indicate the spread of a received signal, it is referred to hereinafter as RDS (Root-mean-square Delay Spread).
For example, consider the case where the size of the delayed part 821 is much smaller compared with the data length (i.e. 32 samples) of the zero suffix interval (ZS interval), which is the target of the overlap-addition, as shown in FIG. 12A. In FIG. 12A, the reference numeral 82 designates noise. Thus, FIG. 12A shows the case where the delay spread is small. In a UWB receiver of a related art, even if the delayed part 821 is small as shown in FIG. 12A, the 32-sample data in the ZS interval is added as shown in FIG. 12B. Therefore, noise 84 in the 32-sample ZS interval is always overlap-added in addition to the delayer part 821, causing the fixed penalty of about 0.97dB to occur, which leads to the degradation of communication properties.
As described above, the present inventor has recognized that the overlap-addition causes a fixed degree of the degradation of communication properties to always occur regardless of the amount of delay spread of a received signal in a UWB receiver of a related art. Such a problem occurs not only in the above-described UWB receiver but occurs generally in a radio receiver that receives a signal where a no-signal interval such as zero suffix or zero prefix is inserted between orthogonal frequency division multiplexed (OFDM) symbols through a radio transmission path and performs overlap-addition on the received signal.
A radio receiver which is disclosed in Japanese Unexamined Patent Application Publication 2002-84332 estimates the delay spread of a received signal, selects either one of fading distortion compensation by pilot symbol insertion or fading distortion compensation by an adaptive equalizer according to the estimated amount of delay spread, and carries out the fading distortion compensation using the selected method. Thus, this technique allows the radio receiver to select the method of frequency equalization in the subcarrier decoder 21 in FIG. 9 according to the amount of delay spread, and it does not address the above problem.